I'm probably a little different than most people. 
The main reason I frame a cel is size/protection. 
I have a number of cels that are too big for the 
largest Itoya portfolios. I felt it was the only 
way to keep them from getting damaged. Otherwise 
they would just sit flat in the same drawer that 
I keep my skteches in. I'm sure the weight of so 
many multilayered large cels would harm the ones 
on the bottom. Framing based only on size has its 
drawbacks. Sometimes I spent more money on the 
framing job (I don't trust my own abilities) than 
I did on the cel. 

I have thought about framing a few favorites, 
since a frame can really enhance a cel's 
appearance, but I haven't done it yet.